The objective of this research is to provide a better understanding and basis for treatment of certain disorders of cardiac rhythm in man due to abnormal impulse formation and (or) conduction. The major emphasis of the research will be on patients with tachycardias arising in the atrium, the atrio-ventricular junction, the ventricle or related to the Preexcitation syndromes (i.e. Wolff-Parkinson-White syndrome, Lown-Ganong-Levine syndrome and variants). Carefully acquired clinical data including ECG and VCG will be obtained on each patient. The patient will then undergo one or more electrophysiologic studies to characterize the mechanism of the arrhythmia, its site of origin, and the response to drug administration and pacing. Many of these patients will have been transferred for surgical therapy of arrhythmias refractory to conventional medical therapy. The patients will undergo mapping of the sequence of excitation of the heart in the operating room. In this way we hope to correlate preoperative findings with ultimate clinical outcome, and to test the hypothesis that surgical inteventions may play a role specifically in the treatment of cardiac rhythm disturbances. We believe a unique feature of this proposal is the coordination in the same patients and by the same investigators of the preoperative and operative electrophysiologic studies.